Can`t Subtract Weight Paint in 2.8?
You can find the brush options in the N panel (on the right of your 3D view) > Tool tab > Brush > Blend, or in the Properties editor > Active Tool tab (the second one) > Brush > Blend. In Weight Paint mode I don't think you can invert the function with any shortcut, ctrl is for sculpting, it would be a nice idea though.